Abstract

ABSTRACT

Background：Osteoarthritis (OA) is a highly prevalent joint degenerative disorder among the older population. The main symptoms of OA are chronic pain, swelling and stiffness of joint. OA histopathology is characterized by cartilage damage, synovial inflammation and remodelling of subchondral bone. Resolvins are endogenous lipid mediators produced from Ω-3 poly-unsaturated fatty acids (PUFAs) during resolution of inflammation. The main biological functions of resolvins include anti-inflammation and resolution of inflammation. Currently, the emerging anti-nociceptive roles of some resolvins have been reported in various models of pain. However, roles of resolvins and the resolvin receptor system on osteoarthritic pain are unknown.

Objectives: This thesis assesses the therapeutic potential of a resolvin precursor on OA pain and investigates the underlying mechanisms of action and resolvin receptor system in OA.

Methods：Monosodium iodoacetate (MIA) and medial meniscus transection (MNX) -induced joint damage was used as models of OA pain. 17(R)-HDoHE (300ng/300μl) or vehicle (1% ethanol in saline, 300μl) was acutely or chronically administered at day 14 post model induction and pain behaviour was measured to determine the analgesic effects of the drug in these models. Haematoxylin and eosin (H&E) staining was used to assess joint histopathology. Gene expression of resolvin receptors, inflammatory cytokines and metabolic enzymes were measured by reverse transcription quantitative polymerase chain reaction (RT-qPCR) in associated tissues from the models and human OA samples.

Results: Pain behaviour and joint histopathology were established in both the MIA and MNX models. Expression of chemokine-like receptor 1(ChemR23) was lower in the synovia and higher in the spinal cord in the MIA model. 15-lipoxygenase (15-LOX) was expressed at a lower level in both synovia and spinal cord in the MIA model. Negative correlations were revealed between synovial ChemR23 expression and pain behaviour at both day 14 and 35 in the MIA model. ChemR23 expression in the spinal cord was positively correlated with pain behaviour at day 35 in the MIA model.

Expression of formyl peptide receptor 2 (ALX), some inflammatory cytokines and metabolic enzymes was lower in the synovia in the MNX model but expression of 5-lipoxygenase-activating protein (FLAP) was higher. Expression of ALX in the synovia was positively correlated with tumor necrosis factor alfa (TNFα), interleukin 1 beta (IL1β) and cyclooxygenase 2(COX2) but negatively correlated with 5-LOX expression in the MIA model. Expression of ALX in the spinal cord was positively correlated with pain behaviour at day 14 but then the converse was true at day 35. Expression of ALX in the spinal cord was negatively correlated with IL6 in the MIA model.

17(R)HDoHE attenuated pain behaviour in both the MIA and MNX models following acute, chronic and discontinuous administration. Effects of acute administration of 17(R)HDoHE on pain behaviour were associated with an up-regulation in the expression of IL6 and decreased 5-LOX expression in the synovia of MIA model. A trend towards down-regulation of pro-inflammatory cytokines and associated enzymes by 17(R)HDoHE was observed in the acute study in the MIA model. Repeated administration of 17(R)HDoHE produced robust and sustained inhibitory effects on pain behaviour, but no change in joint histopathology. Pain behaviour was attenuated when 17(R)HDoHE was administered but returned to levels seen in vehicle treated rats after 7 days after drug cessation.

In human OA samples, expression of ChemR23 was significantly higher than expression of ALX in both synovia and medial tibial plateau. ChemR23 expression was positively correlated with expression of 5-LOX in both synovia and medial tibial plateau and negatively correlated with 15-LOX2 expression in the medial tibial plateau from OA patients. There was a significantly positive correlation between ChemR23 expression and IL6 and 15-LOX1 expression in the medial tibial plateau. In addition, there was a significantly positive correlation between ALX and IL6 and 15-LOX1 expression in both synovia and medial tibial plateau. Expression of ALX, TNFα, IL6, COX2 and 5-LOX in the medial tibial plateau from OA patients was lower, compared to expression in bone from femoral heads obtained from trauma patients.

Conclusions: These findings support anti-nociceptive and anti-inflammatory roles of resolvins and provide evidence that resolvins may be potential novel drugs to treat OA pain